What is Sterile water?
Also called: Sterile Water for Injection, SWFI
Purified water that has been sterilised and contains no preservative, buffer or salt; the default diluent when nothing besides water should enter an experiment.
By the APL Research Team · Updated
Sterile water is purified water that has been sterilised, with nothing added. "Sterile" is a narrow claim: it means no viable microorganisms at the time of packaging. It says nothing about pH, dissolved gases or the bacterial cell-wall fragments known as endotoxin, and once a container is opened it stays sterile only as long as the handling does.
Sterile water versus bacteriostatic water
| Sterile water | Bacteriostatic water | |
|---|---|---|
| Additive | None | 0.9% benzyl alcohol |
| Container use | Single use; the remainder is discarded | Repeated withdrawals from one vial |
| What enters the assay | Water only | Benzyl alcohol in every dilution |
| Main risk | Microbial contamination if a container is reused | Preservative interacting with the peptide or the cells |
| Natural fit | Cell-based work, sensitive sequences, stocks to be aliquoted | Repeated access to one stock over days |
Grades of "pure" water certify different things
Laboratory catalogues list several high-purity waters, and each label guarantees something different:
- Sterile. Free of viable organisms.
- Low-endotoxin or pyrogen-free. Bacterial lipopolysaccharide below a stated limit. This matters for immune-cell and many primary-cell assays, because dead bacteria still shed endotoxin.
- Nuclease-free. Tested for RNase and DNase activity; important for molecular biology, rarely the limiting factor for peptides.
- Ultrapure (Type 1). Very low ions and organic carbon at the point of dispensing, but not sterile in a bottle unless filtered and handled aseptically.
A water can be sterile and still carry endotoxin. For the same reason, passing a solution through a 0.22 µm membrane (sterile filtration) removes bacteria but not endotoxin, which is far smaller than the pores.
Worked example: does the vehicle change the medium?
Adding 50 µL of a water-based stock to 5 mL of a culture medium at about 290 mOsm/kg dilutes the medium's solutes by about 1%, to roughly 287 mOsm/kg. At a 1:10 dilution the drop is about 9%, to roughly 264 mOsm/kg. Concentrated stocks that allow large dilution factors keep the water vehicle's effect on osmolality negligible, and the vehicle control (the same volume of sterile water) accounts for whatever remains.
Practical points
- pH. Unbuffered water in contact with air absorbs carbon dioxide and sits near pH 5.6. A peptide dissolved in it sets the solution's pH through its own ionisable groups and counter-ions, so a sequence that is borderline soluble may behave differently in water than in a buffer.
- No second chances. An opened ampoule or vial is used for one reconstitution and the remainder discarded.
- Storage of the stock. Nothing in the solution suppresses microbial growth, so stocks made in sterile water are best split into single-use aliquots and frozen.
The choice between diluents is covered in bacteriostatic water versus sterile water, and aseptic handling in the sterile technique guide.